The de novo synthesis of nicotinic acid mononucleotide from quinolinic acid and phosphoribosyl pyrophosphate (PPRP) is common to plant, bacterial and mammalian systems. This reaction is catalyzed by the enzyme quinolinate phosphoribosyltransferase (QPRT). QPRT belongs to the family of enzymes which are involved in the biosynthesis of purine, pyrimidine and pyridine nucleotides as well as the aromatic amino acids, histidine and tryptophan. To date, the three-dimensional structure of any one of this group of enzymes has not been elucidated. In order to understand the relationship between structure and function of this enzyme, we have isolated QPRT. We intend to collect high resolution data to above 3.0 w resolution.